This invention relates to captive screws of the type generally used to attach a first panel to a second panel, frame or other surface wherein it is desired to keep the fastener in position on the first panel without loose items of hardware. The captive screw is mounted to the first panel such that the screw stays attached to the panel even when the threads of the screw are fully disengaged from a threaded hole in the second panel to which the first panel is attached.
The present invention is directed toward a new and improved process for manufacturing such a captive screw of a type which has a knob, a screw, and a ferrule. The captive screw is captivated on a panel by the ferrule and, as the screw is screwed into a second panel, the knob telescopically retracts over the ferrule. A spring may be used which urges the knob and integral screw to its fully retracted position when the captive screw is not attached to the second panel, thereby withdrawing the threads of the screw from the area of the lower panel.
The present invention is directed to a process for manufacturing such a captive screw wherein the three major components of the captive screw; the knob, the ferrule, and the screw may each be manufactured by cold forming.
In a typical cold forming process, a slug or billet is deposited within a solid die having a shape and a size corresponding to the desired article to be formed. Force is then longitudinally applied to the slug to cause the metal to flow and conform its shape to that of the die cavity. The article is then removed from the die cavity by pushing it axially out of the die cavity. A series of several dies may be used to incrementally move the material to the final desired shape.
Certain aluminum alloys as well as certain stainless steel alloys are conducive to cold forming. In the present invention, the knob is preferably made from an aluminum alloy, the ferrule is made from an aluminum alloy or carbon or stainless steel, depending on the panel fastening means used with the screw, and the screw itself is made from stainless steel.
When manufacturing in quantities, cold forming of components can offer substantial advantages over other forms of manufacturing items of metal such as machining. While the initial costs associated with setting up a cold forming manufacturing process can be high, including considerable expenses related to engineering and design obstacles, immense time savings with substantial cost savings can be achieved when sufficiently high quantities of captives screws are required. Due to difficulties in cold-forming all of the major components of this type of retractable captive screw, cold forming of the knob and ferrule of the captive screw has not heretofore been achieved. The present invention is for a process that overcomes these difficulties.